You are correct; that is "Merrily We Roll Along," (the OBC, of course!!!!!!!!) but not "The Hills Of Tomorrow." The number actually pictured is the first act finale, "Now, You Know!"
However, its joyous, hopeful moves, call to mind "The Hills Of Tomorrow," a song of hope and yearning, as I get nearer, like Julie as Maria, to stepping out from the abbey. Personally, my mood is mixed; joy and excitement are in there, but so are some darker emotions.
Here is the show's original opening, which, of course, includes the song!!!!!!!!!!